1. Field of the Invention
The invention involves a method for the pyrolysis of hydrocarbons or oxygenised fuels (alcohol, ETB, MTB, . . . ) for the production of a hydrogen-rich gas and possibly for certain applications of carbon monoxide (CO). In particular, but not exclusively, it applies to the conversion of a fuel into a hydrogen-rich gas for fuel cells either low temperature cells of the proton exchange membrane type (PEM) or high temperature cells or molten carbonate fuel cells (MCFC) or solid oxide fuel cells (SOFC).
The expression <<net calorific value>> or NCV is hereafter defined. The calorific value is defined as the quantity of heat given off by the full combustion of the unit of fuel considered. The net calorific value excludes the heat given off, the water condensation heat remaining in vapour state at the end of combustion.
2. Description of the Prior Art
In general, fuel cells are known to be electrochemical systems directly converting chemical energy into electrical energy. The theoretical yield is very high and the sub-products pollute very little. In addition, originally in space missions, fuel cells have demonstrated their efficacy, reliability and longevity. These qualities confirm the value of fuel cells in the production of portable electricity (several hundred watts to several kilowatts) or permanent or on board electricity (2 kilowatts to 200 kilowatts). Therefore, fuel cells represent a possible alternative to thermal engines in a great many of their uses. They can also be used in the creation of co-generation boilers.
Hydrogen fuel may come from a pressurised or cryogenic tank. However, given the safety problems involved in the storage of hydrogen, it is sensible to obtain it from a fuel (hydrocarbon or alcohol) where the hydrogen is released along when needed.
For this purpose, different chemical reactions may be used: vaporeforming, partial oxidation and pyrolysis. Each of these reactions can be activated thermally and/or by means of a catalyst.
The pyrolysis of hydrocarbons not only releases hydrogen but also carbon and other products with a considerable calorific value. This means that the production of hydrogen by pyrolysis only has a sufficient yield if the energy content of the carbon and other co-products is valorised. This situation differs from that of vaporeforming since this method releases the hydrogen contained in the fuel as well as the hydrogen contained in the water. The same is true of partial oxidation when it is coupled with a shift stage.
The reactions are:
Vaporeforming:C3H8 + 6H2O → 3CO2 + 10H2Partial oxidation +C3H8 + 3/2O2 → 3CO + 4H2shift:CO + H2O → CO2 + H2 (×3)Pyrolysis:C3H8 + 3/2O2 + 3H2O → 3CO2 + 7H2C3H8 → 3C + 4H2
However, the experiments carried out with these different methods show that the formation of non negligible quantities of carbon monoxide is inevitable as soon as the oxygen is introduced either directly or in water vapour form.
Therefore, the reformers using partial oxidation or vaporeforming generally include a high or low temperature <<shift>> unit (CO recycling), a vapour generator and a drying unit to remove excess vapour. The conversion of CO is thereby difficult, expensive and cumbersome.
The invention begins with the finding that pyrolysis can be used to eliminate these stages since it occurs without any source of oxygen thereby preventing the formation of CO. In addition, the partial pressure of hydrogen in the gas formed is comparatively higher than in the other methods given, for example, the absence of nitrogen coming from the air used in partial oxidation.
In order to use the advantages of the method related to the absence of oxygen molecules, the choice fuel is a hydrocarbon (methane, propane, butane) or a blend of hydrocarbons. However, an alcohol may be considered as choice in cases where the production of synthesis gases, mixtures of H2 and CO, is required.
Different methods for the use of pyrolysis reactions of hydrocarbons have been recommended by several authors.
In U.S. Pat. No. 5,899,175, Manikowski et al. recommend a hybrid system consisting of a catalytic pyrolysis reactor producing a hydrogen-rich gas and a blend of fuel residues. The hydrogen-rich gas supplies a fuel cell to produce electricity and the blend of fuel residues is burned in a combustion engine to produce mechanical power.
This method allows for the processing of different liquid fuels derived from petroleum. These fuels may be linear or branched alcanes with at least five carbon atoms as well as all types of commercial fuels such gas, kerosene, . . .
The operating conditions chosen as such that only 20% of the hydrogen contained in the fuel is converted into di-hydrogen form.
Poirier et al. recommend a pyrolysis method producing the catalytic decomposition of natural gas into a hydrogen-rich gas and carbon (M. G. Poirier, C. Sapundzhiev, Catalytic decomposition of natural gas to hydrogen for fuel cell applications, Int. J. Hydrogen Energy, vol. 22, No 4, 1997, 429-433). The authors suggest the use of hydrogen-rich gas to supply a PEM fuel cell. The catalytic bed on which the carbon formed during the pyrolysis reaction is deposited is then regenerated by burning the carbon with air. In order to operate in a steady state in spite of the alternate sequences of pyrolysis and regeneration of the catalytic bed, the authors recommend a concept based on the use of two alternating reactors. The first one operates in pyrolysis conditions to produce a hydrogen-rich gas while the second regenerates the catalytic bed by oxidation of the carbon. The structure of the catalytic bed is organised to leave a sufficient dead volume to allow for the accumulation of a large quantity of carbon.
It should be noted that 45% of the net calorific value (NCV) of the natural gas remains in the carbon. Moreover, the pyrolysis of methane, the main component in natural gas, is endothermic and requires about 12% of the NCV of the natural gas. The authors therefore recommend the use of the energy released by the combustion of the carbon to provide the heat required for the decomposition of the natural gas.
In addition, it is necessary to mention that the catalytic bed produces secondary parasite reactions. In fact, the production of CO is observed during the pyrolysis phase while there is no oxygen supply. This emission probably is due to the partial reduction of oxides present in the catalyst and formed during the regeneration phase.
Now, it we want to supply a PEM fuel cell with the hydrogen-rich gas produced by this system, it is necessary to eliminate the CO since over 10 ppm of CO prevents the operation of the cell, since the anode catalyst made of platinum contained is very sensitive to this pollutant. This purification may be carried out by a catalytic methanation method. A methanation reactor therefore has to be placed upstream from the PEM fuel cell on its hydrogen supply circuit.
A system of propane pyrolysis very similar to that of Poirier et al. was recommended by the German Ledjeff-Hey team. It mainly differs by the type of hydrocarbon processed (K. Ledjeff-Hey, V. Formanski, Th. Kalk, J. Roes; Compact Hydrogen Production Systems for Solid Polymer Fuel Cells, J. Power Sources, 71, 1998, 199-207) (K. Ledjeff-Hey, Th. Kalk, J. Roes; Catalytic cracking of propane for hydrogen production for PEM fuel cells, 1998 Fuel Cell Seminar, Palm Springs, Calif. 1998).
The pyrolysis reactions described above present serious defects and deficiencies.
1) In the energy balances:
In fact, pyrolysis methods reveal an intrinsic difficulty: the net calorific value (NCV) of the hydrogen produced is generally of the same magnitude, or even lower than that of the carbon and other pyrolysis residues. There is a resultant problem in the management and valorisation of the energy available in the carbon and other pryolysis residues. If this problem is not suitably dealt with, the global energy balance of the system can only be very low and therefore unacceptable.
2) In the sources of energy required for pyrolysis:
The heat available to heat and decompose the fuel within the pyrolysis reactor may come from three different sources:                the combustion of the carbon during the regeneration sequence,        the combustion of other pyrolysis residues,        the combustion of non-burnt gases leaving the fuel cell.        
These three combustions take place within different chambers: the oxidation of solid carbon takes place inside the reactor in regeneration phase. The combustion of gas emissions may take place in a combustion chamber. It is therefore necessary to plan for very efficient heat exchange structures to provide the efficacy of the heat transfer between the three reaction chambers: reactor during pyrolysis, reactor during regeneration and burner.
3) In the catalyst:
The use of catalytic beds in the two reactors (pyrolysis and regeneration) raises a great many problems: reduced efficacy and ageing of the catalyst, thermal inertia of the reactor, generation of CO during the pyrolysis phase, cost of the system, . . .
4) In the uses:
The supra systems described were designed for the following applications:                hybrid generation of electricity by means of a fuel cell and mechanical energy by means of a combustion engine (U.S. Pat. No. 5,899,175),        generation of electricity in a fuel cell (M. G. Poirier, C. Sapundzhiev, Catalytic decomposition of natural gas to hydrogen for fuel cell applications, Int. J. Hydrogen Energy, vol. 22, No 4, 1997, 429-433), (K. Ledjeff-Hey, V. Formanski, Th. Kalk, J. Roes; Compact Hydrogen Production Systems for Solid Polymer Fuel Cells, J. Power Sources, 71, 1998, 199-207), (K. Ledjeff-Hey, Th. Kalk, J. Roes, Catalytic cracking of propane for hydrogen production for PEM fuel cells, 1998 Fuel Cell Seminar, Palm Springs, Calif. 1998).        